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At which point in the 20th century did the idea arise that the 21st century would begin on 1st January 2000?

The first day of the 19th century is recorded as having been celebrated on 1st January 1801, and the first day of the 20th century on 1st January 1901, which shows that a new century was regarded as commencing on the 1st day of the "01" year, at least up until the beginning of the 20th century.

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    As I remember, there was a lot of argument about when the new millennium would start right up to and after January 1st 2000, such as This Scientific American article
    – Steve Bird
    Jan 2 at 11:02
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    Simple: Most people never gave the issue much thought; and the calendar rolling over like a car odometer has great visual appeal. Jan 2 at 11:11
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    If this question is closed, I would vote to reopen because the lack of understanding of the historical background is the reason why this misunderstanding exists in the first place. Jan 2 at 12:33
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    Required Watching: Seinfeld
    – axsvl77
    Jan 2 at 16:51
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    The confusion regarding the turn of the millenium is a (very) minor plot point in John Brunners "Stand on Zanzibar", so this was a trope as early as 1968. Jan 4 at 8:51

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Because the concept of the "21st century AD" is less useful than that of the "century of the 2000s" (or 2100x or whatever). It's entirely arbitrary.

To an excellent first approximation the only interesting thing about the starting date of the 21st century is bragging rights: "I know there was no Year Zero. Nyah!" It has no religious significance (even if it mattered exactly when He was born, Christ wasn't born in the Year 1.) It has no geological or meteorological significance. Nor astronomical, nor geopolitical, nor legal. (Possibly astrological?) It barely has any historical significance since AD dating wasn't used in the ancient world.

Because we're people and as such have a unremitting lust for round numbers, we mark anniversaries that are round when expressed in base 10 (and why has that any special significance?) so they end in zeros or are simple fractions of numbers which end in zeroes.

For most people the date odometer turning over from --99 to --00 feels a lot more significant than the count-of-years-elapsed-since-the-Year-One passing --99.

(It's true that people speak of things like the "nineteenth century" but I suspect most people, if they have to think about what years that was, follow the algorithm "Well, let's see, it ought to be the century of the 1900s, but I gotta remember to subtract one. Ok, the century of the 1800s." If it weren't for the widespread use of things like "the Sixteenth Century" to denote historical eras, the distinction would be limited to the occasional trivia contest.)

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This is because ancient Roman counting mentality. Latin language has "inclusive counting" features. Keep in mind that historical use of century counting was created by Renaissance latin-speaking European scholars.

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When did the popular idea begin that the 21st century would commence on 1st January 2000?

One can only speculate the when, but the likely reason is when writing dates with Roman numbers became uncommon.

People then often forgot that Roman numbers had no zero.

Since the Anno Domini calendar was introduced at a time when Roman numbers were common place (525), it started with year 1 and not 0 (for which there was no digit).

The last year of the first century (each century being 100 years) was therefore 100. The first year of the second century 101.


Roman numerals - Zero.
"Place-keeping" zeros are alien to the system of Roman numerals - however the actual number zero (what remains after 1 is subtracted from 1) was also missing from the classical Roman numeral system. The word nulla (the Latin word meaning "none") was used to represent 0, although the earliest attested instances are medieval. For instance Dionysius Exiguus used nulla alongside Roman numerals in a manuscript from A.D. 525.

The Anno Domini dating system was devised in 525 by Dionysius Exiguus to enumerate the years in his Easter table.

Year zero - Wikipedia

A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year 1 BC is followed directly by year AD 1.
...

1-to-0 decade.
A rarer approach groups years from the beginning of the AD calendar era to produce successive decades from a year ending in a 1 to a year ending in a 0, with the years 1–10 described as "the 1st decade", years 11–20 "the 2nd decade", and so on; later decades are more usually described as 'the Nth decade of the Mth century' (using the strict interpretation of 'century')

Start and end of centuries.
According to the strict construction, the 1st century AD began with AD 1 and ended with AD 100, the 2nd century spanning the years 101 to 200, with the same pattern continuing onward. In this model, the n-th century starts with the year that ends with "01", and ends with the year that ends with "00"; for example, the 20th century comprises the years 1901 to 2000 in strict usage.

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    Or, you could just say the 1st Century was only 99 years long.
    – Spencer
    Jan 2 at 14:36
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    @Spencer: The short first century. :-)
    – Lucian
    Jan 2 at 14:45
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    Even without the Roman numeral issues the idea of a ‘year 0’ would have just been stupid. It was year 1 because it was the first year of the calendar, not because it had been a year since it started. Jan 2 at 17:53
  • @Spencer One could also state the world is flat. But it's not, just as the Anno Domini calendar year system starts with one and each of it's decades having 10 years each with each of it's century's having 10 decades (using the 1-to-0 decade and strict century construction). Jan 2 at 17:58
  • @MarkJohnson 1-to-0 is called "a rarer approach" than 0-to-9 by the very article you cite. It's all empty pedantry.
    – Spencer
    Jan 2 at 21:01

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